A thermosetting resin is widely used as injection molding, dipping, laminating or molding material in the preparation of various resin products such as electric insulating materials, construction materials, coating compositions, adhesives and the like. Recently, these products are often used under very severe conditions, and with the change in such use conditions, far improved properties and especially heat resisting property, are required of such thermosetting resin.
Since a plastic material having both isocyanurate and oxazolidone group was reported in German Patent Publication (unexamined) 3,223,153, special attention has been directed to this heat resisting novel resin. Such resin is usually prepared by the reaction of polyepoxide compound and polyisocyanate compound and sometimes referred to as polyoxazolidone resin because of having an oxazolidone ring formed by the reaction of epoxy group and isocyanate group. This oxazolidone ring forming reaction is usually carried out in the absence of catalyst at comparatively higher temperatures or in the presence of catalyst at a moderate temperature. Typical catalysts used are tertiary amines (e.g. imidazole, hexamethylenetetramine and the like), quaternary ammonium salts (e.g. iodotetraethylammonium and the like), complex of Lewis acid with base material (e.g. complex of aluminium chloride with pyrrolidone, complex of aluminium chloride with phosphoric ester and the like), lithium halide, complex of lithium halide with phosphor compound (e.g. complex of lithium bromide with tributyl phosphine oxide and the like) alkyl esters of organophosphone acid or phosphoric acid, ammonium salts of acidic esters of organophosphone acid or phosphoric acid and the like. Recently, Matuda et al proposed in J. Org. Chem. 51, (12), 2177-2184 (1986), Chemistry Letters, 1963-1966 (1986) organotin halide-Lewis base complexes and organostibonium halides as novel catalysts for effecting oxazolidone ring formation.
In the case of reaction between monoepoxide and monoisocyanate compounds, the desired oxazolidone compounds may be obtained by using such catalysts and adopting higher temperatures and longer reaction times. However, in the reactions between polyepoxide compounds and polyisocyanate compounds, the situations are more complicated and many by-products are produced together with oxazolidone compounds.
Among them, the main by-products are isocyanurate formed by trimerization of isocyanates and polyether formed by homopolymerization of epoxides. Once the trimerization of isocyanates occurs, there results materials with extremely higher functionalities which give highly crosslinked, brittle polymer. Therefore in the reactions between polyepoxide compounds and poly-isocyanate compounds, it is essential that the trimerization reaction be controlled by all means. However, most of the heretofore proposed catalysts are effective not only for the formation of oxazolidone structure but also for the isocyanurate formation reaction, and polyoxazolidone products obtained by the known methods include 30 mole % or more of by-produced isocyanurates.
When a cured coating is prepared by using a composition comprising polyepoxide and polyisocyanate compounds, thus formed coating is mostly composed of the resins crosslinked with isocyanurate, urea bondings and polyoxazolidone crosslinkings are very few. This may be due to the fact that since various reactions are forced to occur in a very thin layer of the applied composition, they are liable to be affected by the action of moisture in surrounding air, causing hydrolysis of isocyanate groups and inactivation of catalyst used. Therefore, some measure has to be taken in obtaining a cured coating mainly of the resin crosslinked with oxazolidone ring bridge by using a resinous composition comprising polyepoxide and polyisocyanate.
In the case of molded products, the abovementioned moisture effect is not so important. However, with the increase in epoxide content, there occurs severe exothermic reactions, resulting in undesired scorching of the molded product. Such thermal degradation is pointed out as one of the questions still awaiting a good solution for molding use of the abovesaid composition. In such application, some measure has to be taken to react polyepoxide and polyisocyanate under mild condition to give polyoxazolidone containing the least amount of trimerized isocyanurate.
It is, therefore, an object of the invention to provide a thermosetting resinous composition capable of resulting in a high quality plastic having polyoxazolidone groups by the reaction of polyepoxide and polyisocyanate.
Another object of the invention is to provide a method for forming a cured coating of the resin crosslinked with oxazolidone ring bridge by using a coating composition consisting essentially of polyepoxide and polyisocyanate, with the least affect of moisture in surrounding air.
A further object of the invention is to provide a method for forming a molded product mainly composed of the resins crosslinked with oxazolidone ring bridge, containing the least amount of trimerized isocyanurate, by reacting polyepoxide and polyisocyanate under mild conditions.